1. Field of the Invention
The present invention generally relates to a navigation device, and more particularly to a navigation device having an improved information indicating function for navigation.
2. Description of the Related Art
There is a self-sustaining navigation device, which is a position measuring device for use in movable bodies, such as automobiles, airplanes and ships. Such a self- sustaining navigation device obtains a two-dimensional displacement (vector quantity) of a movable body by using azimuth data from an azimuth sensor and speed data from a speed sensor, and adds the two-dimensional displacement to a reference point. In this manner, the current position of the movable body can be identified. For example, the navigation device installed in an automobile operates as follows.
Namely, a predetermined number of pulses are generated while one turn of a drive shaft is completed. The distance between a reference point and the current position is calculated by obtaining the total number of pulses generated during the time the automobile moves the distance. The distance is multiplied by a distance correction coefficient in order to obtain a traveling distance.
A GPS (Global Positioning System) type navigation device is a position measuring device using a satellite. The GPS type navigation device receives electric waves emitted from three or more than GPS satellites, and measures the position of a receiving point by referring to pseudo-distance data including a time offset of a receiver based on the distance between each GPS satellite and the receiving point, and position data indicating each GPS satellite. The position of the receiving point is indicated on a display.
The navigation device is provided with, for example, a current position operation unit, an information processing unit, and a display unit.
The operation of this kind of navigation device is as following. Namely, the current position operation unit generates data indicating the current position of an automobile in which the navigation device is installed. The information processing unit reads, map data concerning an area in which the obtained current position is located. Then, the information processing unit generates graphic data from the map data and the data indicating the current position outputted by the current position operation unit. The display unit receives the graphic data from the information processing unit and displays the map and the current position. In this manner, the driver or passenger of the automobile understands the current position on the map displayed on the display unit.
However, the above explained type of navigation device has the following disadvantage. The picture plane of the display unit has a limited size, and it is required that pieces of information (characters, map symbols and the like) displayed thereon have sizes visible to the naked eye. Hence, there is a limit regarding the area displayable on the picture plane. In particular, when a part of the area on the picture plane is enlarged, it is difficult to understand the current position from the information concerning only the enlarged area displayed on the display unit.
The above-mentioned navigation device has another disadvantage described below. A diverging direction sign (traffic information sign) showing areas in the diverging directions is provided immediately before an intersection. However, if the driver or passenger misses seeing the diverging direction sign, the driver cannot obtain information indicated by the diverging direction sign from the navigation device.